How do I override context for bpy.ops.mesh.loopcut?
A custom context needs to be passed as positional argument, not keyword argument.

bpy.ops.mesh.loopcut(override, number_cuts=...,) # not override=override!


Macros follow the same pattern:

bpy.ops.mesh.loopcut_slide(override, MESH_OT_loopcut={"number_cuts": ...,})


What the Macro bpy.ops.mesh.loopcut_slide() specifically needs as context is a 3D View area (Area.type == 'VIEW_3D'), the window region of that 3D View (usually Area.regions[-1]) and the active object to be a mesh and in edit-mode.